Astronomy Picture of the Day
Index - Nebulae: Emission Nebulae

APOD: 2005 September 30 - IC 1396 H-Alpha Close-Up Explanation:
Clouds of glowing hydrogen
gas mingle ominously with dark dust lanes in this
close-up of IC 1396, an active star
forming region some
2,000 light years away in the constellation Cepheus.
In this and other similar
emission
nebulae, energetic
ultraviolet light
from a hot young star strips electrons from the surrounding
hydrogen atoms.
As the electrons and atoms recombine they emit longer
wavelength, lower energy light in a well known characteristic pattern
of bright spectral lines.
At visible wavelengths, the strongest
emission line in this pattern is in the red part of the spectrum and is
known as "Hydrogen-alpha" or just
H-alpha.
Part of IPHAS,
a survey of H-alpha emission in our Milky Way Galaxy,
this
image spans about 20 light-years and highlights
bright, dense regions within IC 1396, likely sites where
massive new stars are born.

APOD: 2004 January 15 - An Orion Deep Field Explanation:
Adrift 1,500 light-years away in one of
the night sky's
most recognizable
constellations, the glowing
Orion Nebula and the dark
Horsehead Nebula are contrasting
cosmic vistas.
They both appear in this stunning composite digital image
assembled from over 20 hours of data that includes exposures
filtered to record emission from hydrogen
atoms.
The view reveals extensive
nebulosities associated with
the giant Orion Molecular
Cloud
complex, itself hundreds of light-years across.
The magnificent emission region, the
Orion
Nebula (aka M42), lies at the
upper right of the picture.
Immediately to its left are a cluster of
of prominent bluish
reflection nebulae sometimes called
the Running Man.
The Horsehead
nebula appears as a dark cloud, a small silhouette
notched against the long red glow at the lower left.
Alnitak is the easternmost star in Orion's belt and
is seen as the brightest star to the left of the Horsehead.
Below Alnitak is the
Flame Nebula, with clouds of
bright emission and dramatic dark dust lanes.
Fainter tendrils of glowing hydrogen gas are easily traced
throughout the region in
this
Orion deep field.